With split-type conjugate fibers composed of a combination of the same homologous series of resins such as polyolefin-based resins, inherently, the resin components are likely to be strongly bonded to each other at the interface due to the combined resins being highly compatible. Accordingly, such split-type conjugate fibers have lower splittability than those composed of a combination of incompatible polymers such as, for example, a split-type conjugate fiber composed of a combination of a polyester based resin and a polyolefin-based resin and a split-type conjugate fiber composed of a combination of a polyester-based resin and a polyamide-based resin. Accordingly, various attempts are made to improve the splittability of split-type conjugate fibers composed of a combination of polyolefin-based resins. For example, Patent Document 1 discloses a polyolefin-based split-type conjugate fiber composed of polyolefin-based resins having a Rockwell hardness of 60 or greater. Patent Document 2 discloses a polyolefin-based split-type conjugate fiber in which at least one component is blended with a hydrophilic component. Patent Document 3 discloses a polyolefin-based split-type conjugate fiber that is composed of different polyolefin-based resins and in which a component containing a (meth)acrylic acid metal salt and a component that does not contain a (meth)acrylic acid metal salt are adjacent to each other in a cross section of the fiber in order to enhance splittability. Patent Document 4 discloses a polyolefin-based split-type conjugate fiber in which the hollow ratio is 5 to 40%, the ratio between the average external circumferential arc length W of the fiber made of one component and the average thickness L from the hollow portion to the external circumferential portion of the fiber is 0.25 to 2.5, and the melt flow rate (MFR rate) of the two components is defined. Patent Document 5 discloses a polyolefin-based split-type conjugate fiber having a cross section that is formed into a specific flat shape by an external stress.
Patent Documents 1 to 5 mentioned above propose various combinations of polyolefin-based resins such as, for example, a combination of polypropylene (PP) and polyethylene (PE), a combination of polypropylene (PP) and ethylene-propylene copolymer (EP), and a combination of polyethylene and ethylene-propylene copolymer. However, polyolefin-based split-type conjugate fibers composed of a combination of polyolefin-based resins with good splittability and a fineness for use in applications that require fibers having an even smaller fineness and a high level of splittability such as, for example, various types of wiping fiber assemblies such as personal and/or objective wipers, fiber assemblies for battery separators for use in various types of secondary batteries such as lithium ion batteries and nickel-metal hydride batteries, and fiber assemblies for filtration layers for use in various types of filters such as cartridge filters and laminate filters, have not yet been obtained.